1. Field of the Invention
This invention relates to a developing roller which is used in, e.g., electrophotographic apparatus such as copying machines and laser beam printers, and to an electrophotographic process cartridge and an electrophotographic image forming apparatus which make use of the developing roller.
2. Related Background Art
Conventionally, in electrophotographic apparatus or electrostatic recording apparatus, such as copying machines and laser beam printers, a pressure developing method is known as a developing method in which a non-magnetic one-component developer is fed to, e.g., a photosensitive drum which is holding a latent image thereon and the developer is made to adhere to the latent image (electrostatically charged image) on the photosensitive drum to render the latent image visible. According to this method, any magnetic material is not required, and hence the image forming apparatus can be made simple or compact with ease or toners can be made up as color toners with ease.
In this developing method, a developing roller holding a toner (non-magnetic one-component developer) thereon is brought into contact with a latent image bearing member holding an electrostatic latent image thereon, such as a photosensitive drum, to make the toner adhere to the latent image to perform development. Hence, the developing roller must be formed of a conductive elastic member. In recent years, the performance required of this developing roller has come very rich in variety. For example, as disclosed in Japanese Patent Application Laid-open No. H10-213965, it is proposed to add a charge control agent to an elastic layer of a single-layer roller formed of an elastic material.
However, where the non-magnetic one-component developer is used in conjunction with conventional developing rollers, it is difficult to control the charging of toner on the developing rollers, and problems concerning uniformity of charging and running stability of charging have not completely been solved, where image defects such as ghosts may occur. In this case, it is a difficult subject to obtain high-grade images that can achieve high image density especially in a low-temperature and low-humidity environment (15° C., 10% RH).